The answer is A: had been planning.
The form of verb with had + been + verb + ing is called Past Continuous or Progressive tense, and it is used to express an activity in progress recent to another time or activity in the past (Her eyes were red because she had been crying) or to emphasize the duration of an activity that was in progress before another activity or time in the past (He finally came at five o'clock. They had been waiting for him since three-thirry.)